Fuel cells generate electrical current by consuming hydrogen and oxygen. Hydrogen fuel may be supplied to the fuel cell from a hydrogen generator, such as a steam reformer coupled with a palladium membrane hydrogen separator. The reformer is provided for producing hydrogen from a conventional fuel such as natural gas, propane or methanol. The reformer and hydrogen separator produces a substantially pure hydrogen stream in response to a given feed rate of methanol (or other fuel). This process is inherently slow for larger reformers with significant thermal mass. In a fuel cell system, this pure hydrogen gas produced by the hydrogen generator is consumed by the fuel cell, which is typically “dead-ended” (i.e. hydrogen enters and is consumed and there is no path for excess hydrogen to exit directly—this may include systems that recirculate hydrogen). One challenge in mating the two technologies is that since there is no path for excess hydrogen to escape, production rates of hydrogen from the hydrogen generator must match the consumption rate of hydrogen in the fuel cell otherwise any imbalance will manifest as pressure fluctuations in the hydrogen feed lines between the generator and the fuel cell. Pressure fluctuations may have a negative effect on such a system, for example:                Increasing pressure: An increase in the hydrogen pressure will increase the back pressure on the hydrogen separator which relies on a significant pressure drop to produce a substantially pure hydrogen output stream. This will therefore reduce the hydrogen production rate and will also force excess hydrogen rich gas into the reformer oxidation reactor, potentially leading to an overheating scenario.        Decreasing pressure: A decrease in hydrogen pressure will potentially negatively affect the output of the fuel cell, since fuel cells typically rely on a specific pressure drop across the stack to ensure optimum reaction rates. Even in the case where a hydrogen regulator is used to stabilize this pressure, if the pressure drops below a minimum acceptable level, the pressure may be unacceptable for normal fuel cell operation.        
Dead-ended hydrogen fuel cells also generally incorporate a hydrogen purge system which temporarily opens up the “dead-ended” stack to purge contaminants and water which build in concentration during fuel cell operation. This purge briefly and instantaneously requires excess hydrogen from the hydrogen generator which generally cannot be supplied quick enough. The result is a sudden negative pressure fluctuation. Because the purge relies on hydrogen pressure to blow out contaminants, a negative pressure fluctuation may seriously decrease the effectiveness of the purge, which may in turn trigger further purging.
Although some pressure fluctuation can be tolerated, it is advantageous to keep the fluctuations to a minimum and ensure the pressure is substantially stable about a selected operating point.
In addition to maintaining stable hydrogen feed pressure, in certain fuel cell systems it is desirable to run the fuel cell in a quasi-steady state mode (i.e. operating within discrete current output steps) in response to the state of charge of the battery (or other energy storage device). Such a hybrid fuel cell/battery system is described, for example, in Applicant's co-pending U.S. application Ser. No. 09/957,360, now U.S. Pat. No. 6,534,950 the disclosure of which is incorporated herein by reference. Under this strategy the fuel cell will be required to maintain a constant current output for extended durations and then quickly change to a different output level as dictated by the battery state of charge or other external parameters. As current set-points go up, the response time of the fuel cell is not critical (but does affect the recharge time and depth of discharge of the battery) whereas, when set-points go down, the fuel cell must respond quickly so as not to excessively charge and thereby overshoot the voltage threshold of the battery.
This strategy, in combination with the hydrogen feed pressure stabilization requirement, requires the hydrogen generator to operate in a stable output mode for extended periods and quickly load follow on current set-point changes while maintaining a transient response profile directly correlated to the desired transient profile of the fuel cell output current.
The need has therefore arisen for a method and system for effectively integrating the hydrogen processing control requirements of the fuel cell and the hydrogen generator to minimize the negative consequences of hydrogen pressure fluctuations.